This invention was made during the performance of Contract No. DE-AC03-90SF18495 with the United States Department of Energy by Westinghouse Electric Corporation, Pittsburgh, Pennsylvania.
This invention relates to nuclear reactors and more particularly to a passive cooling system for a steel containment vessel used to house a nuclear reactor, the steam generators and associated equipment. In U.S. Pat. No. 5,049,353 issued Sep. 17, 1991 to Westinghouse Electric Corporation, there is described a passive containment cooling system used to cool the steel containment vessel of a nuclear power plant in the event of a failure or event which results in the release of energy from the water cooled reactor. The cooling system described in this patent is the passive heat sink that removes all core decay heat from the steel containment vessel safely and rapidly and prevents any excessive pressure build-up within the steel containment vessel.
As further described in the above-mentioned U.S. Pat. No. 5,049,353, the passive containment cooling system uses a combination of natural air circulation between the steel containment vessel and the concrete shield building and gravity fed cooling water to provide the necessary cooling and heat sink. In the event of a failure of any of the primary active reactor cooling systems or an accident, the passive containment cooling system can be utilized to provide sufficient cooling to maintain and ensure containment vessel integrity.
The gravity fed cooling water used in this passive containment cooling system is derived from a large (350,000 gallon) annular tank built into the roof of the concrete shield building that surrounds the steel containment vessel, and when needed the water is allowed to flow by gravity all over the outside surfaces of the steel containment shell. Because the water is drained by the natural force of gravity, pumps and human operator action are not necessary to provide the necessary cooling effect. The cooling water storage tank is designed to provide the cooling water requirement for several days, after which additional water could be added to the tank. If no additional water is provided, the air cooling would be sufficient to remove residual heat from the steel containment vessel.
In order to provide the most efficient heat removal from the outside surface of the steel containment vessel, the passive cooling water should flow over a large portion of the containment vessel outer surface in a thin film to maximize the cooling effect of the water. Even, uniform distribution of water over the surface of the steel containment vessel is difficult due to the fact that a large steel containment vessel will still have a number of surface deviations, weld joints and other uneven areas that cause the cooling water to "channel" or otherwise not flow over the surface in a thin uniform film. These surface deviations and variations are large compared to the thickness of the desired water film. It has also been observed that if "channeling" occurs near the top of the steel containment vessel, the channel will continue on down the side of the vessel and thereby create a large area that will not be effectively cooled.
Prior attempts to provide for a uniform film of cooling water on the curved surface of the steel containment vessel have improved, but not completely solved the problem. As described in the above-mentioned U.S. Pat. No. 5,049,353, one approach to the problem was to coat the outside surface of the steel containment vessel with a special heat conductive wettable zinc-based paint. While this paint improved the flow of the cooling water as compared to a bare steel surface, and channeling still occurred.